diff --git a/ESP32_CAN_Logger-a.ino b/ESP32_CAN_Logger-a.ino
index a623672..516c59a 100644
--- a/ESP32_CAN_Logger-a.ino
+++ b/ESP32_CAN_Logger-a.ino
@@ -3368,6 +3368,7 @@ void setup() {
     // WebSocket 시작
     webSocket.begin();
     webSocket.onEvent(webSocketEvent);
+    webSocket.enableHeartbeat(15000, 3000, 2);  // ping 15초, pong 3초, 재시도 2회
     
     // ★★★ 웹 서버 라우팅 (중요!) ★★★
     server.on("/", HTTP_GET, []() {
@@ -3399,27 +3400,236 @@ void setup() {
     });
 
     server.on("/download", HTTP_GET, []() {
-        if (server.hasArg("file")) {
-            String filename = "/" + server.arg("file");
-            
-            if (SD_MMC.exists(filename)) {
-                File file = SD_MMC.open(filename, FILE_READ);
-                if (file) {
-                    String displayName = server.arg("file");
-                    server.sendHeader("Content-Disposition", 
-                        "attachment; filename=\"" + displayName + "\"");
-                    server.sendHeader("Content-Type", "application/octet-stream");
-                    server.streamFile(file, "application/octet-stream");
-                    file.close();
-                } else {
-                    server.send(500, "text/plain", "Failed to open file");
-                }
-            } else {
-                server.send(404, "text/plain", "File not found");
-            }
-        } else {
+        if (!server.hasArg("file")) {
             server.send(400, "text/plain", "Bad request");
+            return;
         }
+        
+        String filename = "/" + server.arg("file");
+        
+        Serial.println("\n========================================");
+        Serial.println("📥 다운로드 준비");
+        Serial.println("========================================");
+        
+        // ⭐ 1단계: 모든 로깅 완전 중지
+        bool wasLogging = loggingEnabled;
+        if (wasLogging) {
+            loggingEnabled = false;
+            delay(200);
+            Serial.println("⏸ 모든 로깅 중지");
+        }
+        
+        // ⭐ 2단계: 모든 SD 관련 Task 중단
+        if (sdWriteTaskHandle != NULL) {
+            vTaskSuspend(sdWriteTaskHandle);
+            delay(100);
+            Serial.println("⏸ SD 쓰기 Task 중단");
+        }
+        if (webTaskHandle != NULL) {
+            vTaskSuspend(webTaskHandle);
+            delay(50);
+            Serial.println("⏸ 웹 업데이트 Task 중단");
+        }
+        
+        // ⭐ 3단계: SD Mutex 획득
+        if (xSemaphoreTake(sdMutex, pdMS_TO_TICKS(10000)) != pdTRUE) {
+            if (webTaskHandle != NULL) vTaskResume(webTaskHandle);
+            if (sdWriteTaskHandle != NULL) vTaskResume(sdWriteTaskHandle);
+            if (wasLogging) loggingEnabled = true;
+            server.send(503, "text/plain", "SD card busy");
+            Serial.println("✗ SD Mutex 획득 실패");
+            return;
+        }
+        
+        // ⭐ 4단계: SD 카드 재마운트 (1-bit 모드로)
+        Serial.println("🔄 SD 카드 재마운트 중...");
+        SD_MMC.end();
+        delay(200);
+        
+        // 1-bit 모드로 재시작 (더 안정적)
+        if (!SD_MMC.setPins(SDIO_CLK, SDIO_CMD, SDIO_D0)) {
+            Serial.println("✗ SD_MMC.setPins() 실패");
+            xSemaphoreGive(sdMutex);
+            if (webTaskHandle != NULL) vTaskResume(webTaskHandle);
+            if (sdWriteTaskHandle != NULL) vTaskResume(sdWriteTaskHandle);
+            if (wasLogging) loggingEnabled = true;
+            server.send(500, "text/plain", "SD remount failed");
+            return;
+        }
+        
+        if (!SD_MMC.begin("/sdcard", true)) {  // true = 1-bit mode
+            Serial.println("✗ SD 카드 1-bit 재마운트 실패");
+            xSemaphoreGive(sdMutex);
+            if (webTaskHandle != NULL) vTaskResume(webTaskHandle);
+            if (sdWriteTaskHandle != NULL) vTaskResume(sdWriteTaskHandle);
+            if (wasLogging) loggingEnabled = true;
+            server.send(500, "text/plain", "SD 1-bit mode failed");
+            return;
+        }
+        
+        Serial.println("✓ SD 카드 1-bit 모드 활성화");
+        delay(100);
+        
+        // 파일 존재 확인
+        if (!SD_MMC.exists(filename)) {
+            Serial.println("✗ 파일 없음");
+            
+            // 4-bit 모드로 복구
+            SD_MMC.end();
+            delay(100);
+            SD_MMC.setPins(SDIO_CLK, SDIO_CMD, SDIO_D0, SDIO_D1, SDIO_D2, SDIO_D3);
+            SD_MMC.begin("/sdcard", false);
+            
+            xSemaphoreGive(sdMutex);
+            if (webTaskHandle != NULL) vTaskResume(webTaskHandle);
+            if (sdWriteTaskHandle != NULL) vTaskResume(sdWriteTaskHandle);
+            if (wasLogging) loggingEnabled = true;
+            server.send(404, "text/plain", "File not found");
+            return;
+        }
+        
+        File file = SD_MMC.open(filename, FILE_READ);
+        if (!file) {
+            Serial.println("✗ 파일 열기 실패");
+            
+            // 4-bit 모드로 복구
+            SD_MMC.end();
+            delay(100);
+            SD_MMC.setPins(SDIO_CLK, SDIO_CMD, SDIO_D0, SDIO_D1, SDIO_D2, SDIO_D3);
+            SD_MMC.begin("/sdcard", false);
+            
+            xSemaphoreGive(sdMutex);
+            if (webTaskHandle != NULL) vTaskResume(webTaskHandle);
+            if (sdWriteTaskHandle != NULL) vTaskResume(sdWriteTaskHandle);
+            if (wasLogging) loggingEnabled = true;
+            server.send(500, "text/plain", "Failed to open file");
+            return;
+        }
+        
+        size_t fileSize = file.size();
+        String displayName = server.arg("file");
+        
+        Serial.printf("📥 다운로드 시작: %s (%u bytes)\n", displayName.c_str(), fileSize);
+        
+        // 헤더 전송
+        server.setContentLength(fileSize);
+        server.sendHeader("Content-Disposition", "attachment; filename=\"" + displayName + "\"");
+        server.sendHeader("Content-Type", "application/octet-stream");
+        server.sendHeader("Connection", "close");
+        server.send(200, "application/octet-stream", "");
+        
+        // 512바이트 섹터 단위 전송
+        const size_t CHUNK_SIZE = 512;
+        uint8_t *buffer = (uint8_t*)heap_caps_aligned_alloc(32, CHUNK_SIZE, MALLOC_CAP_DMA);
+        
+        if (!buffer) {
+            buffer = (uint8_t*)malloc(CHUNK_SIZE);
+        }
+        
+        if (!buffer) {
+            Serial.println("✗ 버퍼 할당 실패");
+            file.close();
+            
+            // 4-bit 모드로 복구
+            SD_MMC.end();
+            delay(100);
+            SD_MMC.setPins(SDIO_CLK, SDIO_CMD, SDIO_D0, SDIO_D1, SDIO_D2, SDIO_D3);
+            SD_MMC.begin("/sdcard", false);
+            
+            xSemaphoreGive(sdMutex);
+            if (webTaskHandle != NULL) vTaskResume(webTaskHandle);
+            if (sdWriteTaskHandle != NULL) vTaskResume(sdWriteTaskHandle);
+            if (wasLogging) loggingEnabled = true;
+            return;
+        }
+        
+        size_t totalSent = 0;
+        bool downloadSuccess = true;
+        WiFiClient client = server.client();
+        unsigned long lastPrint = millis();
+        
+        while (file.available() && totalSent < fileSize && downloadSuccess) {
+            // SD 읽기
+            size_t bytesRead = file.read(buffer, CHUNK_SIZE);
+            
+            if (bytesRead == 0) {
+                Serial.printf("✗ 읽기 실패 (위치: %u)\n", totalSent);
+                downloadSuccess = false;
+                break;
+            }
+            
+            // WiFi 전송
+            if (!client.connected()) {
+                Serial.println("✗ 클라이언트 연결 끊김");
+                downloadSuccess = false;
+                break;
+            }
+            
+            size_t totalWritten = 0;
+            while (totalWritten < bytesRead && client.connected()) {
+                size_t written = client.write(buffer + totalWritten, bytesRead - totalWritten);
+                totalWritten += written;
+                if (written == 0) delay(5);
+            }
+            
+            totalSent += bytesRead;
+            
+            // 진행상황 (1초마다)
+            if (millis() - lastPrint > 1000) {
+                float percent = (totalSent * 100.0) / fileSize;
+                Serial.printf("📤 %.1f%% (%u/%u)\n", percent, totalSent, fileSize);
+                lastPrint = millis();
+            }
+            
+            yield();
+        }
+        
+        free(buffer);
+        file.close();
+        
+        Serial.println("========================================");
+        if (downloadSuccess && totalSent == fileSize) {
+            Serial.printf("✓ 완료: %u bytes (100.0%%)\n", totalSent);
+        } else {
+            Serial.printf("⚠ 불완전: %u/%u bytes (%.1f%%)\n", 
+                        totalSent, fileSize, (totalSent * 100.0) / fileSize);
+        }
+        Serial.println("========================================");
+        
+        // ⭐ SD 카드를 4-bit 모드로 복구
+        Serial.println("🔄 SD 카드 4-bit 모드로 복구...");
+        SD_MMC.end();
+        delay(200);
+        
+        if (SD_MMC.setPins(SDIO_CLK, SDIO_CMD, SDIO_D0, SDIO_D1, SDIO_D2, SDIO_D3)) {
+            if (SD_MMC.begin("/sdcard", false)) {  // false = 4-bit mode
+                Serial.println("✓ SD 카드 4-bit 모드 복구 완료");
+            } else {
+                Serial.println("⚠ 4-bit 모드 복구 실패, 1-bit 유지");
+                SD_MMC.setPins(SDIO_CLK, SDIO_CMD, SDIO_D0);
+                SD_MMC.begin("/sdcard", true);
+            }
+        }
+        
+        xSemaphoreGive(sdMutex);
+        
+        // Task 재개
+        if (webTaskHandle != NULL) {
+            vTaskResume(webTaskHandle);
+            Serial.println("▶ 웹 업데이트 Task 재개");
+        }
+        if (sdWriteTaskHandle != NULL) {
+            vTaskResume(sdWriteTaskHandle);
+            Serial.println("▶ SD 쓰기 Task 재개");
+        }
+        
+        // 로깅 재개
+        if (wasLogging) {
+            loggingEnabled = true;
+            Serial.println("▶ 로깅 재개");
+        }
+        
+        Serial.println("\n");
     });
     
     server.begin();
@@ -3441,7 +3651,7 @@ void setup() {
     xTaskCreatePinnedToCore(serialRxTask, "SERIAL_RX", 6144, NULL, 5, &serialRxTaskHandle, 0);
     xTaskCreatePinnedToCore(serial2RxTask, "SERIAL2_RX", 6144, NULL, 5, &serial2RxTaskHandle, 0);  // ⭐ Serial2
     xTaskCreatePinnedToCore(txTask, "TX", 4096, NULL, 3, NULL, 1);
-    xTaskCreatePinnedToCore(webUpdateTask, "WEB_UPDATE", 10384, NULL, 2, &webTaskHandle, 0);  // ⭐ 10240 → 16384
+    xTaskCreatePinnedToCore(webUpdateTask, "WEB_UPDATE", 18384, NULL, 2, &webTaskHandle, 1);  // ⭐ 10240 → 16384
     xTaskCreatePinnedToCore(sdMonitorTask, "SD_MONITOR", 4096, NULL, 1, NULL, 1);
     
     if (timeSyncStatus.rtcAvailable) {